The present invention relates to methods of increasing abiotic stress tolerance and/or biomass in plants and, more particularly, to plants expressing exogenous abiotic stress-tolerance genes.
Abiotic stress (also referred to as “environmental stress”) conditions such as salinity, drought, flood, suboptimal temperature and toxic chemical pollution, cause substantial damage to agricultural plants. Most plants have evolved strategies to protect themselves against these conditions. However, if the severity and duration of the stress conditions are too great, the effects on plant development, growth and yield of most crop plants are profound. Furthermore, most of the crop plants are very susceptible to abiotic stress (ABS) and thus necessitate optimal growth conditions for commercial crop yields. Continuous exposure to stress causes major alterations in the plant metabolism which ultimately lead to cell death and consequently yield losses. Thus, despite extensive research and the use of sophisticated and intensive crop-protection measures, losses due to abiotic stress conditions remain in the billions of dollars annually (1,2).
The following summarizes the implications of exemplary abiotic stress conditions.
Problems associated with drought. A drought is a period of abnormally dry weather that persists long enough to produce a serious hydrologic imbalance (for example crop damage, water supply shortage, etc.). While much of the weather that we experience is brief and short-lived, drought is a more gradual phenomenon, slowly taking hold of an area and tightening its grip with time. In severe cases, drought can last for many years and can have devastating effects on agriculture and water supplies. With burgeoning population and chronic shortage of available fresh water, drought is not only the number one weather related problem in agriculture, it also ranks as one of the major natural disasters of all time, causing not only economic damage, but also loss of human lives. For example, losses from the US drought of 1988 exceeded $40 billion, exceeding the losses caused by Hurricane Andrew in 1992, the Mississippi River floods of 1993, and the San Francisco earthquake in 1989. In some areas of the world, the effects of drought can be far more severe. In the Horn of Africa the 1984-1985 drought led to a famine that killed 750,000 people.
Problems for plants caused by low water availability include mechanical stresses caused by the withdrawal of cellular water. Drought also causes plants to become more susceptible to various diseases (Simpson (1981). “The Value of Physiological Knowledge of Water Stress in Plants”, In Water Stress on Plants, (Simpson, G. M., ed.), Praeger, N.Y., pp. 235-265).
In addition to the many land regions of the world that are too arid for most if not all crop plants, overuse and over-utilization of available water is resulting in an increasing loss of agriculturally-usable land, a process which, in the extreme, results in desertification. The problem is further compounded by increasing salt accumulation in soils, as described above, which adds to the loss of available water in soils.
Problems associated with high salt levels. One in five hectares of irrigated land is damaged by salt, an important historical factor in the decline of ancient agrarian societies. This condition is only expected to worsen, further reducing the availability of arable land and crop production, since none of the top five food crops—wheat, corn, rice, potatoes, and soybean—can tolerate excessive salt.
Detrimental effects of salt on plants are a consequence of both water deficit resulting in osmotic stress (similar to drought stress) and the effects of excess sodium ions on critical biochemical processes. As with freezing and drought, high saline causes water deficit; the presence of high salt makes it difficult for plant roots to extract water from their environment (Buchanan et al. (2000) in Biochemistry and Molecular Biology of Plants, American Society of Plant Physiologists, Rockville, Md.). Soil salinity is thus one of the more important variables that determines where a plant may thrive. In many parts of the world, sizable land areas are uncultivable due to naturally high soil salinity. To compound the problem, salination of soils that are used for agricultural production is a significant and increasing problem in regions that rely heavily on agriculture. The latter is compounded by over-utilization, over-fertilization and water shortage, typically caused by climatic change and the demands of increasing population. Salt tolerance is of particular importance early in a plant's lifecycle, since evaporation from the soil surface causes upward water movement, and salt accumulates in the upper soil layer where the seeds are placed. Thus, germination normally takes place at a salt concentration much higher than the mean salt level in the whole soil profile.
Problems associated with excessive heat. Germination of many crops is very sensitive to temperature. A gene that would enhance germination in hot conditions would be useful for crops that are planted late in the season or in hot climates. Seedlings and mature plants that are exposed to excess heat may experience heat shock, which may arise in various organs, including leaves and particularly fruit, when transpiration is insufficient to overcome heat stress. Heat also damages cellular structures, including organelles and cytoskeleton, and impairs membrane function [Buchanan et al. (2000) in Biochemistry and Molecular Biology of Plants, American Society of Plant Physiologists, Rockville, Md. Heat shock may produce a decrease in overall protein synthesis, accompanied by expression of heat shock proteins. Heat shock proteins function as chaperones and are involved in refolding proteins denatured by heat.
Heat stress often accompanies conditions of low water availability. Heat itself is seen as an interacting stress and adds to the detrimental effects caused by water deficit conditions. Evaporative demand exhibits near exponential increases with increases in daytime temperatures and can result in high transpiration rates and low plant water potentials [Hall et al. (2000) Plant Physiol. 123: 1449-1458]. High-temperature damage to pollen almost always occurs in conjunction with drought stress, and rarely occurs under well-watered conditions. Thus, separating the effects of heat and drought stress on pollination is difficult. Combined stress can alter plant metabolism in novel ways; therefore understanding the interaction between different stresses may be important for the development of strategies to enhance stress tolerance by genetic manipulation.
Problems associated with excessive chilling conditions. The term “chilling sensitivity” has been used to describe many types of physiological damage produced at low, but above freezing, temperatures. Most crops of tropical origins, such as soybean, rice, maize, and cotton are easily damaged by chilling. Typical chilling damage includes wilting, necrosis, chlorosis or leakage of ions from cell membranes. The underlying mechanisms of chilling sensitivity are not completely understood yet, but probably involve the level of membrane saturation and other physiological deficiencies. For example, photoinhibition of photosynthesis (disruption of photosynthesis due to high light intensities) often occurs under clear atmospheric conditions subsequent to cold late summer/autumn nights. For example, chilling may lead to yield losses and lower product quality through the delayed ripening of maize. Another consequence of poor growth is the rather poor ground cover of maize fields in spring, often resulting in soil erosion, increased occurrence of weeds, and reduced uptake of nutrients. A retarded uptake of mineral nitrogen could also lead to increased losses of nitrate into the ground water. By some estimates, chilling accounts for monetary losses in the United States (US) behind only to drought and flooding.
Water deficit is a common component of many plant stresses. Water deficit occurs in plant cells when the whole plant transpiration rate exceeds the water uptake. In addition to drought, other stresses, such as salinity and low temperature, produce cellular dehydration (McCue and Hanson (1990) Trends Biotechnol. 8: 358-362).
Salt and drought stress signal transduction consist of ionic and osmotic homeostasis signaling pathways. The ionic aspect of salt stress is signaled via the SOS pathway where a calcium-responsive SOS3-SOS2 protein kinase complex controls the expression and activity of ion transporters such as SOS1. The pathway regulating ion homeostasis in response to salt stress has been reviewed recently by Xiong and Zhu (2002) Plant Cell Environ. 25: 131-139.
The osmotic component of salt stress involves complex plant reactions that overlap with drought and/or cold stress responses.
Common aspects of drought, cold and salt stress response have been reviewed recently by Xiong and Zhu (2002) supra). Those include:
(a) transient changes in the cytoplasmic calcium levels very early in the signaling event (Knight, (2000) Int. Rev. Cytol. 195: 269-324; Sanders et al. (1999) Plant Cell 11: 691-706);
(b) signal transduction via mitogen-activated and/or calcium dependent protein kinases (CDPKs; see Xiong et al., 2002) and protein phosphatases (Merlot et al. (2001) Plant J. 25: 295-303; Tahtiharju and Palva (2001) Plant J. 26: 461-470);
(c) increases in abscisic acid levels in response to stress triggering a subset of responses (Xiong et al. (2002) supra, and references therein);
(d) inositol phosphates as signal molecules (at least for a subset of the stress responsive transcriptional changes (Xiong et al. (2001) Genes Dev. 15: 1971-1984);
(e) activation of phospholipases which in turn generate a diverse array of second messenger molecules, some of which might regulate the activity of stress responsive kinases (phospholipase D functions in an ABA independent pathway, Frank et al. (2000) Plant Cell 12: 111-124);
(f) induction of late embryogenesis abundant (LEA) type genes including the CRT/DRE responsive COR/RD genes (Xiong and Zhu (2002) supra);
(g) increased levels of antioxidants and compatible osmolytes such as proline and soluble sugars (Hasegawa et al. (2000) Annu. Rev. Plant Mol. Plant Physiol. 51: 463-499); and
(h) accumulation of reactive oxygen species such as superoxide, hydrogen peroxide, and hydroxyl radicals (Hasegawa et al. (2000) supra).
Abscisic acid biosynthesis is regulated by osmotic stress at multiple steps. Both ABA-dependent and -independent osmotic stress signaling first modify constitutively expressed transcription factors, leading to the expression of early response transcriptional activators, which then activate downstream stress tolerance effector genes.
Based on the commonality of many aspects of cold, drought and salt stress responses, it can be concluded that genes that increase tolerance to cold or salt stress can also improve drought stress protection. In fact this has already been demonstrated for transcription factors (in the case of AtCBF/DREB1) and for other genes such as OsCDPK7 (Saijo et al. (2000) Plant J. 23: 319-327), or AVP1 (a vacuolar pyrophosphatase-proton—pump, Gaxiola et al. (2001) Proc. Natl. Acad. Sci. USA 98: 11444-11449).
Developing stress-tolerant plants is a strategy that has the potential to solve or mediate at least some of these problems. However, traditional plant breeding strategies used to develop new lines of plants that exhibit tolerance to ABS are relatively inefficient since they are tedious, time consuming and of unpredictable outcome. Furthermore, limited germplasm resources for stress tolerance and incompatibility in crosses between distantly related plant species represent significant problems encountered in conventional breeding. Additionally, the cellular processes leading to ABS tolerance are complex in nature and involve multiple mechanisms of cellular adaptation and numerous metabolic pathways (4-7).
Genetic engineering efforts, aimed at conferring abiotic stress tolerance to transgenic crops, have been described in the prior art. Studies by Apse and Blumwald (Curr Opin Biotechnol. 13:146-150, 2002), Quesada et al. (Plant Physiol. 130:951-963, 2002), Holmström et al. (Nature 379: 683-684, 1996), Xu et al. (Plant Physiol 110: 249-257, 1996), Pilon-Smits and Ebskamp (Plant Physiol 107: 125-130, 1995) and Tarczynski et al. (Science 259: 508-510, 1993) have all attempted at generating stress tolerant plants.
In addition, several U.S. patents and patent applications also describe polynucleotides associated with stress tolerance and their use in generating stress tolerant plants. U.S. Pat. Nos. 5,296,462 and 5,356,816 describe transforming plants with polynucleotides encoding proteins involved in cold adaptation in Arabidopsis thaliana, to thereby promote cold tolerance in the transformed plants.
U.S. Pat. No. 6,670,528 describes transforming plants with polynucleotides encoding polypeptides binding to stress responsive elements, to thereby promote tolerance of the transformed plants to abiotic stress.
U.S. Pat. No. 6,720,477 describes transforming plants with a polynucleotide encoding a signal transduction stress-related protein, capable of increasing tolerance of the transformed plants to abiotic stress.
U.S. application Ser. Nos. 09/938,842 and 10/342,224 describe abiotic stress-related genes and their use to confer upon plants tolerance to abiotic stress.
U.S. application Ser. No. 10/231,035 describes overexpressing a molybdenum cofactor sulfurase in plants to thereby increase their tolerance to abiotic stress.
Although the above described studies were at least partially successful in generating stress tolerant plants, there remains a need for stress tolerant genes which can be utilized to generate plants tolerant of a wide range of abiotic stress conditions.
While reducing the present invention to practice, the present inventors have identified through bioinformatic and laboratory studies several novel abiotic stress-tolerance genes, which can be utilized to increase tolerance to abiotic stress and/or biomass, vigor and yield in plants.